Letter series type of questions usually consists of a series of small letters which follow a certain pattern. However, some letters are missing from the series. These missing letters are then given in a proper sequence as one of the alternatives. The candidate is requitred to choose this alternative as the answer.
b _ b _ bb _ _ bbb _ bb _ b
The series is babb / bbab / bbba / bbbb.
Thus, in each sequence, a moves one step forward and b takes its place and finally in the fourth sequence, it is eliminated.